My colleague discovered something very odd while adding text
to a drawing. He inserted a Note and tried to find the font he
needed (a company font for a logo) in the drop-down menu on the
Formatting Toolbar, but it wasn't there.

He was able to change the font later by selecting the Note and
using the Font... dialog under Text Format in the Property Manager.

Why the inconsistency? Is there a way to customize or add to the
drop-down options in the Formatting Toolbar?

I have encountered this very problem. I have installed some
new fonts, both True-Type and Open-Type format, into the
C:\WINDOWS\Fonts folder, but SolidWorks will not list them as
available for notes, annotations, text, anything.

As a double-check, I am able to use my installed fonts in both MS
Word and Open-Office programs, so I'm really at a loss for why
SolidWorks won't open them. Argh!

I believe that the issue that you are having has to do with the font's filetype. While SW supports both TrueType and OpenType fonts, it appears that it only "sees" font files that have a filetype of .ttf. If you check, it is likely that your new files are .otf files, which will be recognized by the OS in Windows/Fonts and can be seen by other apps (e.g. Word, Excel, etc.) without a problem. However they will not appear in any of SW's font lists.

To clarify, the SW fonts (SWAstro, SWComp, etc.) are TrueType fonts (indicated by the "TT" icon) whose font files have a .ttf filetype; Sylfaen is an OpenType font (indicated by the "O" icon), but whose font file also has a .ttf filetype. If you try to install an OpenType font, whose filetype is .otf, SW will not recognize it.